US6750393B2 - Back reflector of solar cells - Google Patents
Back reflector of solar cells Download PDFInfo
- Publication number
- US6750393B2 US6750393B2 US10/177,040 US17704002A US6750393B2 US 6750393 B2 US6750393 B2 US 6750393B2 US 17704002 A US17704002 A US 17704002A US 6750393 B2 US6750393 B2 US 6750393B2
- Authority
- US
- United States
- Prior art keywords
- solar cell
- superprism
- photoactive region
- air voids
- cell according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
- 239000004038 photonic crystal Substances 0.000 claims abstract description 30
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 30
- 239000011148 porous material Substances 0.000 claims description 19
- 238000000034 method Methods 0.000 claims description 18
- 229910021426 porous silicon Inorganic materials 0.000 claims description 13
- 229910052681 coesite Inorganic materials 0.000 claims description 12
- 229910052906 cristobalite Inorganic materials 0.000 claims description 12
- 239000000377 silicon dioxide Substances 0.000 claims description 12
- 229910052682 stishovite Inorganic materials 0.000 claims description 12
- 229910052905 tridymite Inorganic materials 0.000 claims description 12
- 230000000737 periodic effect Effects 0.000 claims description 11
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 10
- 238000000137 annealing Methods 0.000 claims description 9
- 229910052710 silicon Inorganic materials 0.000 claims description 9
- 239000010703 silicon Substances 0.000 claims description 9
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 8
- 229910052739 hydrogen Inorganic materials 0.000 claims description 8
- 239000001257 hydrogen Substances 0.000 claims description 8
- 239000004005 microsphere Substances 0.000 claims description 6
- 238000002310 reflectometry Methods 0.000 description 14
- 239000011800 void material Substances 0.000 description 8
- 238000001228 spectrum Methods 0.000 description 5
- 238000010521 absorption reaction Methods 0.000 description 4
- 229910021419 crystalline silicon Inorganic materials 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 4
- 238000010586 diagram Methods 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 239000011022 opal Substances 0.000 description 3
- 230000003647 oxidation Effects 0.000 description 3
- 238000007254 oxidation reaction Methods 0.000 description 3
- 230000001902 propagating effect Effects 0.000 description 3
- 239000010409 thin film Substances 0.000 description 3
- 230000009466 transformation Effects 0.000 description 2
- 238000007792 addition Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000007812 deficiency Effects 0.000 description 1
- 238000003487 electrochemical reaction Methods 0.000 description 1
- 238000005286 illumination Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000005012 migration Effects 0.000 description 1
- 238000013508 migration Methods 0.000 description 1
- 238000002161 passivation Methods 0.000 description 1
- 238000000059 patterning Methods 0.000 description 1
- 238000001020 plasma etching Methods 0.000 description 1
- 230000006798 recombination Effects 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 235000012431 wafers Nutrition 0.000 description 1
Images
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/1225—Basic optical elements, e.g. light-guiding paths comprising photonic band-gap structures or photonic lattices
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/02—Details
- H01L31/0236—Special surface textures
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/02—Details
- H01L31/0236—Special surface textures
- H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
- H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
- H01L31/0547—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means comprising light concentrating means of the reflecting type, e.g. parabolic mirrors, concentrators using total internal reflection
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
- H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
- H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/52—PV systems with concentrators
Definitions
- the invention relates to the field of solar cells, and in particular to thin crystalline silicon solar cells.
- FIG. 1 shows the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells (shown by line 102 ) and the photon number spectrum calculated from the air mass two (AM2) sun spectrum (shown by line 100 ).
- IQE internal quantum efficiency
- the photon number spectrum 100 consists of various peaks that survive absorption and scattering in the air. Since the Si optical edge is at 1.2 ⁇ m, photons in roughly half of the second peak 104 as well as the third peak 106 are wasted in thin Si solar cells because of the IQE reduction. This results in a low efficiency for current thin Si solar cells (the best reported one is ⁇ 15%, as described in R. Brendel, “ Crystalline Thin - film Silicon Solar Cells from Layer - transfer Processes: a Review ,” Proc. 10 th Workshop on Crystalline Silicon Solar Cell Materials and Processes, ed. by B. L Sopori, 117, 2000.). The efficiency of thin Si solar cells should at least equal the efficiency of bulk Si solar cells, which is 25%.
- the current structure to perform this light trapping is the Lambertian top surface and Al backside electrode.
- the Al electrode has a reflectivity as low as 98%.
- the Lambertian structure has the same internal reflectivity as Al, more than 99% of incident photons escape from cells if they bounce only 100 times between the surfaces.
- light with wavelengths near Si's bandgap must be bounced back and forth more than 1000 times to be fully absorbed in current thin Si cells. This is because optical paths 10 cm or longer are required for 1.2 ⁇ m light to be absorbed in Si and generate electron-hole pairs, while current thin Si solar cells are only about 50 ⁇ m thick.
- the present invention alleviates problems with current light trapping in solar cells by using a photonic crystal as a backside reflector.
- one aspect of the present invention provides for a solar cell that comprises a photoactive region; a Lambertian surface on the topside of the photoactive region; and a photonic crystal on the backside of the photoactive region.
- Another aspect of the present invention provides a method of forming a solar cell that comprises forming a Lambertian surface on a topside of a photoactive region; and forming a photonic crystal on a backside of the photoactive region.
- FIG. 1 illustrates the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells and the photon number spectrum calculated from the air mass two (AM2) sun spectrum;
- IQE internal quantum efficiency
- FIG. 2 illustrates a solar cell, according to one embodiment of the present invention, that uses a one-dimensional photonic crystal as a backside reflector;
- FIGS. 3 a-b show the reflectivity of light for a one-dimensional photonic crystal backside reflector made of 3 pairs of Si/SiO 2 layer stacks;
- FIG. 4 a illustrates a solar cell, according to another embodiment of the present invention, that uses a photonic crystal superprism as a backside reflector;
- FIG. 4 b illustrates the theoretical band diagram of a 3-dimensional photonic crystal formed from a square lattice of air voids for a ratio of air void radius r to periodicity a equal to 0.48;
- FIG. 5 illustrates another embodiment of a solar cell according to the present invention that combines the embodiments of FIGS. 2 and 3;
- FIG. 6 illustrates one method of forming a void array in silicon
- FIG. 7 illustrates a well aligned porous Si microstructure
- FIG. 8 illustrates porous silicon with a periodic variation of the pore diameter with pore depth
- FIG. 9 illustrates electro-statically self-assembled SiO 2 microspheres.
- FIG. 2 illustrates a solar cell 200 according to one embodiment of the present invention.
- Solar cell 200 is preferably a thin Si solar cell.
- Solar cell 200 has a photoactive region 208 with a topside Lambertian surface 204 and a backside photonic crystal 202 formed, for example, from alternating dielectric stacks 206 a and 206 b .
- Photonic crystals are periodic dielectric structures that have a photonic band gap (PBG) that forbids propagation of a certain frequency range of light. Any incident wave in the frequency range arriving onto the crystal will be reflected rather than transmitted. Photonic crystals are described in more detail in J. D. Joannopoulos, R. D. Meade, J. N. Winn, “Photonic Crystals” (Princeton, 1995).
- PBG photonic band gap
- Photonic crystal 202 is a one-dimensional photonic crystal and acts as a “perfect” mirror whose reflectance can be controlled to be more than 99.99%.
- dielectric stacks 206 a and 206 b are made of high index material such as Si ( 206 b ) and SiO 2 ( 206 a ), only four pairs of the layer stacks are needed to realize 99.99% reflectivity.
- FIGS. 3 a-b show the reflectivity of light for a stack made of 3 pairs of Si/SiO 2 layer stacks, where the Si is 80 nm thick and the SiO 2 is 188 nm thick.
- FIG. 3 a illustrates the reflectivity for light normally incident on the stack
- FIG. 3 b illustrates the reflectivity for light incident at 33 degrees.
- Line 302 is the reflectivity of the TM mode
- line 304 is the reflectivity of the TE mode.
- Increasing the backside reflectivity reduces the number of photons lost during light reflection. This allows light to be bounced back and forth more than 100 times before 99% of the photons are lost, which allows more light with longer wavelengths to be absorbed.
- FIG. 4 a illustrates a solar cell 400 according to this embodiment.
- Solar cell 400 is preferably a thin Si solar cell.
- solar cell 400 has a photoactive region 408 with a topside Lambertian surface 404 and a backside photonic crystal superprism 402 .
- Superprism 402 and Lambertian surface 404 are designed so that normal incident light 410 (e.g., from the sun 412 ) is refracted with a momentum parallel to superprism 402 (along the cell plane). This allows it to travel a longer path in solar cell 400 .
- superprism 402 is a 3-dimensional photonic crystal formed from a cubic lattice of air voids.
- no photonic state exists except the M valleys for photons with normalized frequencies ⁇ a/2 ⁇ c between 0.27 and 0.33.
- the M valleys correspond to four equivalent diagonal directions in the square lattice in real space. Thus, the only allowed propagation direction is along the diagonal directions in the square lattice.
- incident light with a normalized frequency between 0.27 and 0.33 that is propagating normal to the superprism's (100) lattice plane or in the ⁇ X direction will be defracted, while light propagating in the ⁇ M direction will be directed along one of the diagonal directions.
- the frequency range over which this occurs can be adjusted by changing the periodicity a of the air voids.
- the light incident ⁇ on superprism 402 should be at a slight angle so that some component is propagating in the ⁇ M direction. This small angle of incidence naturally occurs when normal incident light 410 passes through Lambertian surface 404 . The light will then be directed along one of the diagonal directions as long as the angle has a component in the ⁇ M direction. Which M valley the refracted light follows depends on the initial light momentum, i.e. its incident angle to superprism 402 .
- the combined Lambertian surface 404 and superprism 402 causes the momentum of incident light 410 to be directed parallel to the cell plane. Because the length of a solar cell is typically longer than its thickness, this increases the propagation length of the light, which allows longer wavelengths to be absorbed without sizable losses from leakage. For instance, in current technology the distance in the direction of the M valleys is typically 10 cm for 4 inch wafers, which is equivalent to the absorption length in Si for 1.2 ⁇ m wavelength light.
- FIG. 5 illustrates another embodiment of a solar cell 500 according to the present invention that combines the embodiments of FIGS. 2 and 3.
- Solar cell 500 is preferably a thin Si solar cell.
- solar cell 500 has a photoactive region 508 with a topside Lambertian surface 504 .
- a superprism 502 is formed, for example, as a 3-dimensional photonic crystal formed from a cubic lattice of air voids.
- a photonic crystal 514 is formed under first superprism 502 from, for example, alternating dielectric stacks 506 a and 506 b.
- superprism 502 directs light both inward (along the diagonals internal to superprism 502 ) and outward (along the diagonals leading out of superprism 502 ).
- photonic crystal 514 reflects some of the light that is directed outward back into superprism 502 .
- a number of techniques can be used to form the superprism.
- One method is to transform a deep via-hole to a void array as described by Mizushima et al. in “ Empty - space - in - silicon Technique for Fabricating a Silicon - on - nothing Structure ,” Appl. Phys. Lett. 77, 3290, 2000, which is incorporated herein by reference.
- FIG. 6 shows this technology for the void array formation.
- the via-hole array is fabricated by plasma etching a trench 600 , whose aspect ratio is about 30. Then the trench structure is annealed at 1100° C. for 10 min in H 2 ambient. As illustrated in FIG. 6, the via-hole structure transforms to a void array. This is due to minimization of surface energy with the aid of high surface diffusivity of Si under H 2 ambient.
- the radius r and periodicity a of the voids are controlled by the radius and aspect ratio of via-holes and annealing temperatures.
- the via-holes should be 0.2 ⁇ m in diameter with periodicity of 0.36 ⁇ m. Because of the large index difference (2.5) between air and Si, 4 voids in depth are enough to realize a reflectivity of 99.99%. Thus, the via-holes could be as shallow as 2 ⁇ m in depth. In other words, the aspect ratio needed is only 10.
- the void inner surface acts as a non-radiative recombination center if it is not properly passivated.
- passivation of the void inner surface helps increase the charge collection efficiency. This can be done by internal oxidation, as described in T. Saito et al. “ Microstructure Transformation of Silicon: A Newly Developed Transformation Technology for Patterning Silicon Surfaces using the Surface Migration of Silicon Atoms by Hydrogen Annealing ,” Jpn. J. of Appl. Phys. 39, 5033, 2000, incorporated herein by reference.
- the interface states between Si and SiO 2 should be very low for a grown oxide.
- Another method for fabricating a via-hole array for a superprism is to use electrochemical reactions, or anodic oxidation to form porous Si. This is then followed by hydrogen annealing to form the air voids.
- Grunging, et al. recently showed well aligned porous Si in “ Two - dimensional Infrared Photonic Bandgap Structure Based on Porous Silicon ,” Appl. Phys. Lett. 66, 3254, 1995, incorporated herein by reference. This microstructure is shown in FIG. 7 .
- the via hole dimensions shown are too large, but finer structures are possible by varying doping and current density during anodic oxidation.
- the superprism can be fabricated by forming porous silicon with a periodic index variation along the pores resulting from a periodic variation of the pore diameter with pore depth.
- a technique for forming such porous silicon is described in Schilling et al., “ Three - dimensional Photonic Crystals based on Macroporous Silicon with Modulated Pore Diameter ,” Appl. Phys. Lett. 78, 1180, 2001, which is incorporated herein by reference.
- Schilling et al. achieve the periodic variation of pore diameter with pore depth by periodically modulating backside illumination (and consequently etch current) while the pores grow into the substrate.
- FIG. 8 illustrates a modulated pore structure resulting from the technique of Schilling et al.
- opal consists of SiO 2 spheres in a photonic crystal array with an incomplete PBG.
- fabrication of electro-statically self-assembled SiO 2 microspheres, opal can also be used to form a superprism.
- FIG. 9 illustrates such self-assembled microspheres.
- a superprism with a wider PBG can also be realized by replacing SiO 2 spheres with oxided Si spheres.
Landscapes
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Microelectronics & Electronic Packaging (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Computer Hardware Design (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- Power Engineering (AREA)
- Optics & Photonics (AREA)
- Chemical & Material Sciences (AREA)
- Nanotechnology (AREA)
- Life Sciences & Earth Sciences (AREA)
- Biophysics (AREA)
- Crystallography & Structural Chemistry (AREA)
- Photovoltaic Devices (AREA)
Abstract
A solar cell that has a photoactive region; a Lambertian surface on the topside of the photoactive region; and a photonic crystal on the backside of the photoactive region.
Description
This application claims priority from provisional application Ser. No. 60/300,681 filed Jun. 25, 2001.
The invention relates to the field of solar cells, and in particular to thin crystalline silicon solar cells.
Thin crystalline Si solar cells are attractive because they use small volumes of Si material, and they should prove to be cost effective. However, the short optical path length in crystalline Si solar cells reduces the conversion efficiency of photons to carriers. FIG. 1 shows the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells (shown by line 102) and the photon number spectrum calculated from the air mass two (AM2) sun spectrum (shown by line 100). As shown, an IQE reduction starts at 0.8 μm and goes to zero at ˜1.1 μm, despite the fact that photon wavelengths up to 1.2 μm can yield carrier generation (optical bandgap defined by absorption coefficient α˜10−1 cm−1 at 1.2 μm).
The photon number spectrum 100 consists of various peaks that survive absorption and scattering in the air. Since the Si optical edge is at 1.2 μm, photons in roughly half of the second peak 104 as well as the third peak 106 are wasted in thin Si solar cells because of the IQE reduction. This results in a low efficiency for current thin Si solar cells (the best reported one is ˜15%, as described in R. Brendel, “Crystalline Thin-film Silicon Solar Cells from Layer-transfer Processes: a Review,” Proc. 10th Workshop on Crystalline Silicon Solar Cell Materials and Processes, ed. by B. L Sopori, 117, 2000.). The efficiency of thin Si solar cells should at least equal the efficiency of bulk Si solar cells, which is 25%.
To overcome this deficiency in thin Si solar cells, light is typically bounced between the top and bottom surfaces of the solar cell. The current structure to perform this light trapping is the Lambertian top surface and Al backside electrode. The Al electrode has a reflectivity as low as 98%. Assuming that the Lambertian structure has the same internal reflectivity as Al, more than 99% of incident photons escape from cells if they bounce only 100 times between the surfaces. Yet, light with wavelengths near Si's bandgap must be bounced back and forth more than 1000 times to be fully absorbed in current thin Si cells. This is because optical paths 10 cm or longer are required for 1.2 μm light to be absorbed in Si and generate electron-hole pairs, while current thin Si solar cells are only about 50 μm thick. Thus, it is very difficult to increase the absorption at near band edge wavelengths (near 1.2 μm) by using the structure based on the Lambertian surface and Al reflectors.
The present invention alleviates problems with current light trapping in solar cells by using a photonic crystal as a backside reflector.
Thus, one aspect of the present invention provides for a solar cell that comprises a photoactive region; a Lambertian surface on the topside of the photoactive region; and a photonic crystal on the backside of the photoactive region.
Another aspect of the present invention provides a method of forming a solar cell that comprises forming a Lambertian surface on a topside of a photoactive region; and forming a photonic crystal on a backside of the photoactive region.
FIG. 1 illustrates the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells and the photon number spectrum calculated from the air mass two (AM2) sun spectrum;
FIG. 2 illustrates a solar cell, according to one embodiment of the present invention, that uses a one-dimensional photonic crystal as a backside reflector;
FIGS. 3a-b show the reflectivity of light for a one-dimensional photonic crystal backside reflector made of 3 pairs of Si/SiO2 layer stacks;
FIG. 4a illustrates a solar cell, according to another embodiment of the present invention, that uses a photonic crystal superprism as a backside reflector;
FIG. 4b illustrates the theoretical band diagram of a 3-dimensional photonic crystal formed from a square lattice of air voids for a ratio of air void radius r to periodicity a equal to 0.48;
FIG. 5 illustrates another embodiment of a solar cell according to the present invention that combines the embodiments of FIGS. 2 and 3;
FIG. 6 illustrates one method of forming a void array in silicon;
FIG. 7 illustrates a well aligned porous Si microstructure;
FIG. 8 illustrates porous silicon with a periodic variation of the pore diameter with pore depth;
FIG. 9 illustrates electro-statically self-assembled SiO2 microspheres.
FIG. 2 illustrates a solar cell 200 according to one embodiment of the present invention. Solar cell 200 is preferably a thin Si solar cell. Solar cell 200 has a photoactive region 208 with a topside Lambertian surface 204 and a backside photonic crystal 202 formed, for example, from alternating dielectric stacks 206 a and 206 b. Photonic crystals are periodic dielectric structures that have a photonic band gap (PBG) that forbids propagation of a certain frequency range of light. Any incident wave in the frequency range arriving onto the crystal will be reflected rather than transmitted. Photonic crystals are described in more detail in J. D. Joannopoulos, R. D. Meade, J. N. Winn, “Photonic Crystals” (Princeton, 1995). Photonic crystal 202 is a one-dimensional photonic crystal and acts as a “perfect” mirror whose reflectance can be controlled to be more than 99.99%. When dielectric stacks 206 a and 206 b are made of high index material such as Si (206 b) and SiO2 (206 a), only four pairs of the layer stacks are needed to realize 99.99% reflectivity.
To illustrate the high reflectivity possible, FIGS. 3a-b show the reflectivity of light for a stack made of 3 pairs of Si/SiO2 layer stacks, where the Si is 80 nm thick and the SiO2 is 188 nm thick. FIG. 3a illustrates the reflectivity for light normally incident on the stack, while FIG. 3 b illustrates the reflectivity for light incident at 33 degrees. Line 302 is the reflectivity of the TM mode and line 304 is the reflectivity of the TE mode.
Increasing the backside reflectivity reduces the number of photons lost during light reflection. This allows light to be bounced back and forth more than 100 times before 99% of the photons are lost, which allows more light with longer wavelengths to be absorbed.
However, even using a highly reflective photonic crystal 202, light will not likely be bounced back and forth more than a thousand times without leakage because the topside Lambertian surface 204 has a low reflectivity of ˜98%. Leakage resulting from this low reflectivity reduces the number of photons by nearly 90% for light that has bounced 100 times.
Accordingly, another embodiment of the present invention uses a photonic crystal superprism as a backside reflector to increase the propagation path length of reflected light. A photonic crystal superprism is a photonic crystal with an incomplete photonic bandgap that provides for a large angle refraction of incident light, i.e. it is a directional reflector. FIG. 4a illustrates a solar cell 400 according to this embodiment. Solar cell 400 is preferably a thin Si solar cell. As shown, solar cell 400 has a photoactive region 408 with a topside Lambertian surface 404 and a backside photonic crystal superprism 402. Superprism 402 and Lambertian surface 404 are designed so that normal incident light 410 (e.g., from the sun 412) is refracted with a momentum parallel to superprism 402 (along the cell plane). This allows it to travel a longer path in solar cell 400.
In one embodiment, superprism 402 is a 3-dimensional photonic crystal formed from a cubic lattice of air voids. FIG. 4b shows the theoretical band diagram of such a photonic crystal for a ratio of air void radius r to periodicity a equal to, for example, 0.48, i.e. r/a=0.48. As can be seen, no photonic state exists except the M valleys for photons with normalized frequencies ωa/2πc between 0.27 and 0.33. The M valleys correspond to four equivalent diagonal directions in the square lattice in real space. Thus, the only allowed propagation direction is along the diagonal directions in the square lattice. That is, incident light with a normalized frequency between 0.27 and 0.33 that is propagating normal to the superprism's (100) lattice plane or in the ΓX direction will be defracted, while light propagating in the ΓM direction will be directed along one of the diagonal directions. The frequency range over which this occurs can be adjusted by changing the periodicity a of the air voids.
The light incident λ on superprism 402 should be at a slight angle so that some component is propagating in the ΓM direction. This small angle of incidence naturally occurs when normal incident light 410 passes through Lambertian surface 404. The light will then be directed along one of the diagonal directions as long as the angle has a component in the ΓM direction. Which M valley the refracted light follows depends on the initial light momentum, i.e. its incident angle to superprism 402.
Thus, the combined Lambertian surface 404 and superprism 402 causes the momentum of incident light 410 to be directed parallel to the cell plane. Because the length of a solar cell is typically longer than its thickness, this increases the propagation length of the light, which allows longer wavelengths to be absorbed without sizable losses from leakage. For instance, in current technology the distance in the direction of the M valleys is typically 10 cm for 4 inch wafers, which is equivalent to the absorption length in Si for 1.2 μm wavelength light.
FIG. 5 illustrates another embodiment of a solar cell 500 according to the present invention that combines the embodiments of FIGS. 2 and 3. Solar cell 500 is preferably a thin Si solar cell. As shown, solar cell 500 has a photoactive region 508 with a topside Lambertian surface 504. A superprism 502 is formed, for example, as a 3-dimensional photonic crystal formed from a cubic lattice of air voids. A photonic crystal 514 is formed under first superprism 502 from, for example, alternating dielectric stacks 506 a and 506 b.
In general, superprism 502 directs light both inward (along the diagonals internal to superprism 502) and outward (along the diagonals leading out of superprism 502). As shown, photonic crystal 514 reflects some of the light that is directed outward back into superprism 502.
A number of techniques can be used to form the superprism. One method is to transform a deep via-hole to a void array as described by Mizushima et al. in “Empty-space-in-silicon Technique for Fabricating a Silicon-on-nothing Structure,” Appl. Phys. Lett. 77, 3290, 2000, which is incorporated herein by reference. FIG. 6 shows this technology for the void array formation.
The via-hole array is fabricated by plasma etching a trench 600, whose aspect ratio is about 30. Then the trench structure is annealed at 1100° C. for 10 min in H2 ambient. As illustrated in FIG. 6, the via-hole structure transforms to a void array. This is due to minimization of surface energy with the aid of high surface diffusivity of Si under H2 ambient. The radius r and periodicity a of the voids are controlled by the radius and aspect ratio of via-holes and annealing temperatures.
As previously described, the band diagram illustrated in FIG. 5 was obtained for r/a=0.48. From this, the diameter and period of the voids is calculated to be 0.34 and 0.36 μm, respectively, in order for the structure of FIG. 6 to act as the superprism for a 1.2 μm wavelength. For a ratio between the voids and the via-holes in size of 1.88 (as described in Mizushima et al.), the via-holes should be 0.2 μm in diameter with periodicity of 0.36 μm. Because of the large index difference (2.5) between air and Si, 4 voids in depth are enough to realize a reflectivity of 99.99%. Thus, the via-holes could be as shallow as 2 μm in depth. In other words, the aspect ratio needed is only 10.
In this structure, the void inner surface acts as a non-radiative recombination center if it is not properly passivated. Thus, passivation of the void inner surface helps increase the charge collection efficiency. This can be done by internal oxidation, as described in T. Saito et al. “Microstructure Transformation of Silicon: A Newly Developed Transformation Technology for Patterning Silicon Surfaces using the Surface Migration of Silicon Atoms by Hydrogen Annealing,” Jpn. J. of Appl. Phys. 39, 5033, 2000, incorporated herein by reference. The interface states between Si and SiO2 should be very low for a grown oxide.
Another method for fabricating a via-hole array for a superprism is to use electrochemical reactions, or anodic oxidation to form porous Si. This is then followed by hydrogen annealing to form the air voids. Grunging, et al. recently showed well aligned porous Si in “Two-dimensional Infrared Photonic Bandgap Structure Based on Porous Silicon,” Appl. Phys. Lett. 66, 3254, 1995, incorporated herein by reference. This microstructure is shown in FIG. 7. The via hole dimensions shown are too large, but finer structures are possible by varying doping and current density during anodic oxidation.
Also, the superprism can be fabricated by forming porous silicon with a periodic index variation along the pores resulting from a periodic variation of the pore diameter with pore depth. A technique for forming such porous silicon is described in Schilling et al., “Three-dimensional Photonic Crystals based on Macroporous Silicon with Modulated Pore Diameter,” Appl. Phys. Lett. 78, 1180, 2001, which is incorporated herein by reference. Schilling et al. achieve the periodic variation of pore diameter with pore depth by periodically modulating backside illumination (and consequently etch current) while the pores grow into the substrate. FIG. 8 illustrates a modulated pore structure resulting from the technique of Schilling et al.
Another method of forming a superprism uses electro-statically self-assembled opal. It is known that opal consists of SiO2 spheres in a photonic crystal array with an incomplete PBG. Thus, fabrication of electro-statically self-assembled SiO2 microspheres, opal, can also be used to form a superprism. FIG. 9 illustrates such self-assembled microspheres. A superprism with a wider PBG can also be realized by replacing SiO2 spheres with oxided Si spheres.
Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.
Claims (20)
1. A solar cell comprising:
a photoactive region with a top and a bottom;
a Lambertian surface on the top of the photoactive region; and
a photonic crystal on the bottom of the photoactive region, wherein said photonic crystal comprises a superprism.
2. The solar cell according to claim 1 , wherein the superprism is formed from a cubic lattice of air voids.
3. The solar cell according to claim 2 , wherein the solar cell is a thin silicon solar cell.
4. The solar cell according to claim 3 , wherein the cubic lattice of air voids is formed by hydrogen annealing of etched trenches.
5. The solar cell according to claim 3 , wherein the cubic lattice of air voids is formed by hydrogen annealing of porous silicon.
6. The solar cell according to claim 1 , wherein the superprism is formed from electro-statically self-assembled microspheres of SiO2 or oxided Si.
7. The solar cell according to claim 1 , wherein the superprism is fabricated by forming porous silicon with a periodic index variation along the pores resulting from a periodic variation of the pore diameter with pore depth.
8. A method of forming a solar cell comprising:
forming a Lambertian surface on a top of a photoactive region; and
forming a photonic crystal on a bottom of the photoactive region, wherein said photonic crystal comprises a superprism.
9. The method according to claim 8 , wherein the superprism is formed from a cubic lattice of air voids.
10. The method according to claim 9 , wherein the solar cell is a thin silicon solar cell.
11. The method according to claim 10 , wherein the cubic lattice of air voids is formed by hydrogen annealing of etched trenches.
12. The method according to claim 10 , wherein the cubic lattice of air voids is formed by hydrogen annealing of porous silicon.
13. The method according to claim 8 , wherein the superprism is formed from electro-statically self-assembled microspheres of SiO2 or oxided Si.
14. The method according to claim 8 , wherein the superprism is fabricated by forming porous silicon with a periodic index variation along the pores resulting from a periodic variation of the pore diameter with pore depth.
15. A thin silicon solar cell comprising:
a photoactive region;
a Lambertian surface on top of the photoactive region; and
a superprism on a bottom of the photoactive region;
wherein light that is normally incident on the Lambertian surface is directed by the superprism so as to have momentum parallel to the superprism.
16. The solar cell according to claim 15 , wherein the superprism is formed from a cubic lattice of air voids.
17. The solar cell according to claim 16 , wherein the cubic lattice of air voids is formed by hydrogen annealing of etched trenches.
18. The solar cell according to claim 16 , wherein the cubic lattice of air voids is formed by hydrogen annealing of porous silicon.
19. The solar cell according to claim 15 , wherein the superprism is formed from electro-statically self-assembled microspheres of SiO2 or oxided Si.
20. The solar cell according to claim 15 , wherein the superprism is fabricated by forming porous silicon with a periodic index variation along the pores resulting from a periodic variation of the pore diameter with pore depth.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US10/177,040 US6750393B2 (en) | 2001-06-25 | 2002-06-21 | Back reflector of solar cells |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US30068101P | 2001-06-25 | 2001-06-25 | |
US10/177,040 US6750393B2 (en) | 2001-06-25 | 2002-06-21 | Back reflector of solar cells |
Publications (2)
Publication Number | Publication Date |
---|---|
US20030029496A1 US20030029496A1 (en) | 2003-02-13 |
US6750393B2 true US6750393B2 (en) | 2004-06-15 |
Family
ID=23160153
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US10/177,040 Expired - Fee Related US6750393B2 (en) | 2001-06-25 | 2002-06-21 | Back reflector of solar cells |
Country Status (2)
Country | Link |
---|---|
US (1) | US6750393B2 (en) |
WO (1) | WO2003001609A2 (en) |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20070235072A1 (en) * | 2006-04-10 | 2007-10-11 | Peter Bermel | Solar cell efficiencies through periodicity |
US20090085512A1 (en) * | 2007-09-27 | 2009-04-02 | Motorola, Inc. | Apparatus for charging a battery of a portable electronic device |
US20100035413A1 (en) * | 2005-01-13 | 2010-02-11 | Chung-Hua Li | Active layer for solar cell and the manufacturing method making the same |
US20110017287A1 (en) * | 2008-03-25 | 2011-01-27 | Nicholas Francis Borrelli | Substrates for photovoltaics |
US20120118373A1 (en) * | 2008-01-29 | 2012-05-17 | Emot Co., Ltd. | Silicon solar cell |
TWI408825B (en) * | 2010-09-24 | 2013-09-11 | Univ Nat Chiao Tung | A solar cell apparatus having the transparent conducting layer with the periodic structure |
US20220365259A1 (en) * | 2021-05-17 | 2022-11-17 | Hrl Laboratories, Llc | Modular photonic reflectors |
Families Citing this family (15)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US7310454B2 (en) * | 2004-05-24 | 2007-12-18 | Massachusetts Institute Of Technology | Photonic bandgap modulator, amplifier, demux, and TDM devices |
WO2006078319A1 (en) * | 2005-01-19 | 2006-07-27 | Massachusetts Institute Of Technology | Light trapping in thin film solar cells using textured photonic crystal |
US20070169808A1 (en) * | 2006-01-26 | 2007-07-26 | Kherani Nazir P | Solar cell |
JP5054355B2 (en) * | 2006-11-13 | 2012-10-24 | 株式会社カネカ | Photoelectric conversion device |
TW200919743A (en) * | 2007-10-30 | 2009-05-01 | Aurotek Corp | Dye-sensitized solar cell |
WO2009099071A1 (en) * | 2008-02-04 | 2009-08-13 | The University Of Tokyo | Silicon solar cell |
ES2554770T3 (en) * | 2008-04-18 | 2015-12-23 | Nlab Solar Ab | Dye-sensitized solar cell with one-dimensional photonic crystal |
US20110155215A1 (en) * | 2009-12-31 | 2011-06-30 | Du Pont Apollo Limited | Solar cell having a two dimensional photonic crystal |
WO2011139852A2 (en) * | 2010-04-29 | 2011-11-10 | Skyline Solar, Inc. | Thin film coating pinning arrangement |
WO2011140355A2 (en) * | 2010-05-07 | 2011-11-10 | Applied Materials, Inc. | Oxide nitride stack for backside reflector of solar cell |
JP5590965B2 (en) * | 2010-05-24 | 2014-09-17 | 三菱電機株式会社 | Photovoltaic element module and manufacturing method thereof |
US20120132277A1 (en) * | 2010-11-30 | 2012-05-31 | General Electric Company | Photovoltaic device and method for making |
GB201215344D0 (en) * | 2012-08-29 | 2012-10-10 | Ibm | Light-reflecting grating structure for photvoltaic devices |
US20190296682A1 (en) * | 2018-01-10 | 2019-09-26 | The American University In Cairo | Silicon based mid-ir super absorber using hyperbolic metamaterial |
US20230418051A1 (en) * | 2020-11-20 | 2023-12-28 | Silbat Energy Storage Solutions, S.L. | Monolithic mirror and method for designing same |
Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4918030A (en) * | 1989-03-31 | 1990-04-17 | Electric Power Research Institute | Method of forming light-trapping surface for photovoltaic cell and resulting structure |
US5080725A (en) * | 1987-12-17 | 1992-01-14 | Unisearch Limited | Optical properties of solar cells using tilted geometrical features |
US6130780A (en) * | 1998-02-19 | 2000-10-10 | Massachusetts Institute Of Technology | High omnidirectional reflector |
US6469682B1 (en) * | 1999-05-11 | 2002-10-22 | Agence Spatiale Europeenne | Periodic dielectric structure of the three-dimensional photonic band gap type and method for its manufacture |
-
2002
- 2002-06-21 US US10/177,040 patent/US6750393B2/en not_active Expired - Fee Related
- 2002-06-21 WO PCT/US2002/020028 patent/WO2003001609A2/en not_active Application Discontinuation
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5080725A (en) * | 1987-12-17 | 1992-01-14 | Unisearch Limited | Optical properties of solar cells using tilted geometrical features |
US4918030A (en) * | 1989-03-31 | 1990-04-17 | Electric Power Research Institute | Method of forming light-trapping surface for photovoltaic cell and resulting structure |
US6130780A (en) * | 1998-02-19 | 2000-10-10 | Massachusetts Institute Of Technology | High omnidirectional reflector |
US6469682B1 (en) * | 1999-05-11 | 2002-10-22 | Agence Spatiale Europeenne | Periodic dielectric structure of the three-dimensional photonic band gap type and method for its manufacture |
Non-Patent Citations (3)
Title |
---|
Baba et al, "Photonic Crystal Light Deflection Devices Using the Superprism Effect," IEEE Journal of Quantum Electronics, vol. 38, No. 7, pp. 909-914, Jul. 2002. * |
Gee, "Optically Enhanced Absorption in Thin Silicon Layers Using Photonic Crystals," 29th IEEE Photovoltaic Specialists Conference, pp. 150-153 (2002).* * |
Kosaka et al, "Superprism Phenomena in Photonic Crystals: Toward Microscale Lightwave Circuits," Journal of Lightwave Technology, vol. 17, No. 11, pp. 2032-2038, Nov. 1999.* * |
Cited By (11)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20100035413A1 (en) * | 2005-01-13 | 2010-02-11 | Chung-Hua Li | Active layer for solar cell and the manufacturing method making the same |
US8163589B2 (en) * | 2005-01-13 | 2012-04-24 | Aurotek Corporation | Active layer for solar cell and the manufacturing method making the same |
US20070235072A1 (en) * | 2006-04-10 | 2007-10-11 | Peter Bermel | Solar cell efficiencies through periodicity |
US20090085512A1 (en) * | 2007-09-27 | 2009-04-02 | Motorola, Inc. | Apparatus for charging a battery of a portable electronic device |
US7872442B2 (en) * | 2007-09-27 | 2011-01-18 | Motorola Mobility, Inc. | Apparatus for charging a battery of a portable electronic device |
US20120118373A1 (en) * | 2008-01-29 | 2012-05-17 | Emot Co., Ltd. | Silicon solar cell |
US9052425B2 (en) * | 2008-01-29 | 2015-06-09 | Samwon Fa Co., Ltd. | Silicon solar cell |
US20110017287A1 (en) * | 2008-03-25 | 2011-01-27 | Nicholas Francis Borrelli | Substrates for photovoltaics |
TWI408825B (en) * | 2010-09-24 | 2013-09-11 | Univ Nat Chiao Tung | A solar cell apparatus having the transparent conducting layer with the periodic structure |
US20220365259A1 (en) * | 2021-05-17 | 2022-11-17 | Hrl Laboratories, Llc | Modular photonic reflectors |
US11693160B2 (en) * | 2021-05-17 | 2023-07-04 | Hrl Laboratories, Llc | Modular photonic reflectors |
Also Published As
Publication number | Publication date |
---|---|
WO2003001609A2 (en) | 2003-01-03 |
WO2003001609A3 (en) | 2003-11-27 |
WO2003001609A9 (en) | 2003-10-16 |
US20030029496A1 (en) | 2003-02-13 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US6750393B2 (en) | Back reflector of solar cells | |
US10622498B2 (en) | Microstructure enhanced absorption photosensitive devices | |
US7482532B2 (en) | Light trapping in thin film solar cells using textured photonic crystal | |
US8647903B2 (en) | Method of fabricating antireflective grating pattern and method of fabricating optical device integrated with antireflective grating pattern | |
US20110247676A1 (en) | Photonic Crystal Solar Cell | |
US8896077B2 (en) | Optoelectronic semiconductor device and method of fabrication | |
US20130327928A1 (en) | Apparatus for Manipulating Plasmons | |
US20090253227A1 (en) | Engineered or structured coatings for light manipulation in solar cells and other materials | |
Stepikhova et al. | Light emission from Ge (Si)/SOI self-assembled nanoislands embedded in photonic crystal slabs of various periods with and without cavities | |
US20100236620A1 (en) | Thin film solar cell and method for producing the same | |
US20130014814A1 (en) | Nanostructured arrays for radiation capture structures | |
US11309444B1 (en) | Microstructure enhanced absorption photosensitive devices | |
US20230215962A1 (en) | Microstructure enhanced absorption photosensitive devices | |
US6967112B2 (en) | Three-dimensional quantum dot structure for infrared photodetection | |
US6031951A (en) | Transmission-mode optical coupling mechanism and method of manufacturing the same | |
US20120037208A1 (en) | Thin film solar cell structure | |
WO2018180765A1 (en) | Texture structure manufacturing method | |
Boutami et al. | Photonic crystal-based MOEMS devices | |
WO2018061898A1 (en) | Optical sensor and method for forming same | |
CN113097356B (en) | On-chip light source, preparation method of on-chip light source and optoelectronic device | |
US7812423B2 (en) | Optical device comprising crystalline semiconductor layer and reflective element | |
Gutman et al. | Two‐and three‐dimensional composite photonic crystals of macroporous silicon and lead sulfide semiconductor nanostructures | |
US20220246775A1 (en) | Microstructure enhanced absorption photosensitive devices | |
Lockwood et al. | Visible light from Si/SiO 2 superlattices in planar microcavities | |
CN118103748A (en) | High-efficiency grating coupler for laser source |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
AS | Assignment |
Owner name: MASSACHUSETTS INSTITUTE OF TECHNOLOGY, MASSACHUSET Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:WADA, KAZUMI;KIMERLING, LIONEL C.;TOYODA, NORIAKI;REEL/FRAME:013402/0302;SIGNING DATES FROM 20020830 TO 20021004 |
|
FPAY | Fee payment |
Year of fee payment: 4 |
|
FEPP | Fee payment procedure |
Free format text: PAYOR NUMBER ASSIGNED (ORIGINAL EVENT CODE: ASPN); ENTITY STATUS OF PATENT OWNER: SMALL ENTITY |
|
REMI | Maintenance fee reminder mailed | ||
LAPS | Lapse for failure to pay maintenance fees | ||
STCH | Information on status: patent discontinuation |
Free format text: PATENT EXPIRED DUE TO NONPAYMENT OF MAINTENANCE FEES UNDER 37 CFR 1.362 |
|
FP | Lapsed due to failure to pay maintenance fee |
Effective date: 20120615 |